1. Field of the Invention
The present invention relates to an organic electro luminescence display and, more particularly, to an organic electro luminescence display in which a cathode power line is connected with a cathode electrode by a plurality of contact holes.
2. Description of the Related Art
Typically, organic electro luminescence displays are self-emissive displays that are classified, according to the direction of light emitted from their organic emission layer, as either bottom-emitting types, top-emitting types, or dual-emitting types. The top-emitting type emits light in a direction away from the substrate on which the pixels are arranged, while the bottom-emitting type emits light toward the substrate on which the pixels are arranged. Top-emitting types have a higher aperture ratio than bottom-emitting types.
In the top-emitting type, because the light is emitted from the organic emission layer away from the substrate on which the pixels are arranged, one of the electrodes between which the organic emission layer is interposed should be transparent so that light may be transmitted through it. Typically, the transparent electrode is made of transparent conductive material such as Indium Tin Oxide (ITO). Transparent conductive material has a high resistance value, however, which causes a voltage (IR) drop, thereby creating an inconsistent brightness on the display.
To solve this problem, a technology has been proposed that uses a metallic material as a cathode power line for supplying a cathode voltage to a transparent cathode electrode, which is one of the two electrodes formed on the upper and lower portions of the organic emission layer.
FIG. 1 shows a plan view of a conventional organic electro luminescence display with a cathode power line.
Referring to FIG. 1, a conventional organic electro luminescence display 100 comprises a pixel portion 110 on which a plurality of pixels are arranged, an upper power line 120 on the top, right and left sides of the pixel portion 110 to supply the power supply voltage VDD, a lower power line 130 on the bottom side of the pixel portion 110 to supply the power supply voltage VDD, a scan driver 140 that supplies a scan signal sequentially to pixels of the pixel portion 110, a data driver 150 that supplies a data signal to pixels of the pixel portion 110, and a cathode electrode 160, which is formed to cover all of the pixel portion 110.
The conventional organic electro luminescence display 100 further comprises an external terminal 171, which is used to apply an external voltage to the cathode power line 170. As shown in FIGS. 1 and 2, the cathode power line 170 is connected with the cathode electrode 160 by contact hole 180, thereby connecting the cathode electrode 160 to the external voltage supplied at the external terminal 171.
With this configuration, the conventional organic electro luminescence operates as follows.
The scan driver 140 signal and the data driver 150 signal are transmitted to the pixels in the pixel portion 110. A predetermined level of power supply voltage VDD is supplied from the upper and lower power lines 120, 130 to the pixels in the pixel portion 110, and the cathode voltage is supplied from the cathode power line 170 to the cathode electrode 160 via the contact hole 180. Switching and driving transistors (not shown) provided in each of the pixels arranged in the pixel portion 110 then operate, so that light is emitted from the organic emission layer and transmitted through the cathode electrode 160.
During this operation, the electric current flowing through the cathode power line 170 is concentrated on the edge of the contact hole 180, so that current density is highest at the edge of the contact hole 180. Equipotential lines within the contact hole 180, as shown in FIG. 2, show that the current mobility decreases from the edge to the center of the contact hole 180, and the current is lowest at the center of the contact hole 180.
Hence, the larger the contact hole 180 is, the more its circumference is lengthened, thereby concentrating the current density on its edge. This results in a decrease of current mobility from the edge to the center of the contact hole 180, which in turn results in a voltage (IR) drop and decreases brightness in the display.